1037-92-9Relevant academic research and scientific papers
Modeling the catalyst resting state in aryl tin(IV) polymerizations of lactide and estimating the relative rates of transamidation, transesterification and chain transfer
Chisholm, Malcolm H.,Delbridge, Ewan E.,Gallucci, Judith C.
, p. 145 - 152 (2007/10/03)
The preparation and characterization (IR, 1H, 13C{1H}, 119Sn NMR spectroscopy, elemental analysis and single crystal X-ray structure determination) are reported for Ph3SnOCMe2C(O)OEt (1) and Ph2Sn[OCMe 2C(O)NMe2]2 (2). In the solid state, compound 1 contains four-coordinate tin with evidence for incipient bond formation to the ester oxygen: Sn...O = 2.648(2) A. Compound 2 contains six-coordinate tin in a pseudo-octahedral geometry. The OCMe2C(O) NMe2 groups form cis-chelates with short, ca. 2.03 A, and long, ca. 2.26 A, Sn-O bonds to alkoxide and amide oxygen atoms, respectively. In solution, compound 1 remains four-coordinate but compound 2 exists as an equilibrium mixture of six-coordinate and five-coordinate species as judged by NMR spectroscopy. At -50°C in toluene-d8, the six-coordinate isomer is favored and the NMR data are consistent with the structure observed in the solid state. At +50°C, the NMR data are consistent with a five-coordinate species in which reversible chelation of η2- and η1-OCMe2C(O)NMe2 is fast on the NMR time scale. The molecular structure of 2 and its dynamic solution behavior is proposed to resemble that of Ph2Sn[OCHMeC(O)NMe2] 2 formed in the polymerization of L-lactide by Ph2 Sn(NMe2)2. The high formation tendency of this compound is proposed to be responsible for the preferential formation of cyclic lactide oligomers (LA/2)n by intrachain transesterification, in contrast to polymerizations employing Ph 2Sn(OPri)2, which produce long chains of H-(LA/2)n-OPri where LA = [OCHMeC(O)OCHMeC(O)]. The kinetics of the reactions between Ph3SnX and each of Me 2CHC(O)OMe, Me(MeO)CHC(O)OEt and Ph3SnOCHMeC(O)OEt, have been determined from NMR studies in benzene-d6 where X = NMe 2 or OPri. Similarly, the reaction between Ph 3SnOBut and (p-tolyl)3SnOPri has been followed. The former reactions represent transamidation and transesterification, and the latter models chain transfer. These findings, when compared to the earlier studies of the ring-opening of lactide and its subsequent ring-opening polymerization, indicate that the rate follows the order: chain transfer > ring-opening > ring-opening polymerization > transesterification, although the latter is influenced by the ester end-group.
